Mach-Zehnder and Michelson interferometers are commonly known in optical telecommunication. The first Mach-Zehnder interferometers (MZI) built for balanced receiver applications utilized glass ion exchange waveguides with a 3 dB coupler. These devices were found to have too much polarization dependence. This made biasing the interferometer difficult because the polarization state at the receiver was random and varied with time. This meant that the desired bias point of the interferometer needed to vary with the same dynamics as the input polarization state which is unknown.
It is known, for example from Yonenaga et al., “Dispersion-Tolerant Optical Transmission System Using Duobinary Transmitter and Binary Receiver”, Journal of Lightwave Technology, Vol. 15, No. 8, Aug. 1997, pages 1530-1537, and from Yonenaga et al. U.S. Pat. No. 5,543,952 issued Aug. 6, 1996 and entitled “Optical Transmission System”, to use duobinary code for a modulating signal supplied in push-pull manner to a dual-drive Mach-Zehnder (MZ) type optical intensity modulator in an optical communications system. The use of duobinary code in this manner reduces the signal bandwidth required for a given signal transmission rate, and permits direct detection to recover the original binary signal at an optical receiver. Such an arrangement again requires an external modulator and involves the costs and risks associated therewith especially in an array transmission system. For example, cross-talk of high voltage, high frequency signals among closely spaced electrical circuits presents a significant problem, and modulation using duobinary encoded signals as disclosed by Yonenaga et al. doubles the voltage swings of signals supplied to the modulators, thereby exacerbating this problem.
An alternative duobinary encoding technique is described in International patent application PCT/CA98/00275 by Northern Telecom Limited, published Oct. 8, 1998 under No. WO 98/44635 and entitled “Duobinary Coding And Modulation Technique For Optical Communication Systems”.
The article by Yonenaga et al. referred to above also refers to a dispersion-supported transmission (DST) technique, as disclosed by B. Wedding et al., “10-Gb/s optical transmission up to 253 km Via Standard Single-Mode Fiber Using the Method of Dispersion-Supported Transmission”, Journal of Lightwave Technology, Vol. 12, No. 10, October 1994, pages 1720-1727. The DST technique uses direct modulation of a laser diode with a NRZ binary signal to produce an FSK optical signal, and FM-AM conversion in the dispersive optical fiber with direct detection of the AM component at an optical receiver. Consequently, the DST technique requires the frequency deviation of the FSK optical signal to be adjusted, depending upon the chromatic dispersion of the fiber, to match the group delay between the FSK components to the bit duration. In addition, recovery of the NRZ binary signal from the detected AM component of the converted optical signal requires additional processing, for example by an integrator and a decision circuit.
U.S. Pat. No. 6,473,214 issued Oct. 29, 2002 to Nortel Networks Ltd. (Roberts et al.), describes a method and apparatus for optical signal transmission. The specification of the patent is incorporated by reference therewith. According to the patent, a binary signal is encoded to produce a three-level encoded signal having reduced bandwidth. As shown in FIG. 5 of the Nortel patent, an interference filter is provided preferably in the form of a Mach-Zehnder interferometer having an optical splitter and an optical combiner. These define two optical paths. A relative or differential optical signal delay between these two optical paths causes in turn a constructive interference between the two optical paths.
U.S. Pat. No. 5,917,638 to Lucent describes a Mach-Zehnder modulator with a 1-bit delay for the purpose of encoding information in a light beam, applicable to a signal transmitter.
It is an object of the invention to provide a passive interferometric filter for decoding incoming data for a signal receiver.
It is another object of the invention to provide such filter with a relatively high signal-to-noise ratio (SNR) to enable relatively low level signals at the receiver, and with a relatively low polarization loss (PDL).
In the balanced receiver application, it is desirable to have one arm's optical path length to be different from the other arm by n bit lengths. Said another way, it is desirable to delay one path by an integral number of bits. This allows a pair of (not necessarily adjacent) bits to be compared (combined) and allows for common mode noise reduction.
It is also possible to transmit information in a modified duo-binary form so that there is more information content for a given transmission bandwidth. This allows more spectral efficiency or more information to be packed in a DWDM channel at a given transmission rate.